Combining the global distributions of black carbon (BC) mass concentration given by GADS (Global Aerosol Data Set), The direct radiative forcing due to BC have been calculated with an improved radiative transfer model under clear sky in both winter and summer. The results show that the radiative forcing by BC is positive at the tropopause and negative at the surface, which is different from the positive radiative forcing of greenhouse gases through the whole atmosphere. The reason is explained theoretically in this paper. The global mean forcing values are 0.085 W/m2 for winter and 0.155 W/m2 for summer at the tropopause, while they are -0.37 W/m2 for winter and -0.63 W/m2 for summer at the surface. Although the radiative forcing due to BC is highly dependent on the optical properties of itself and its concentration in the atmosphere, surface albedo and zenith angle also influence the forcing greatly. It is pointed out in this work that the absolute values of radiative forcing due to BC at both the tropopause and the surface increase linearly with the cosine of zenith angle and the surface albedo. The distribution of BC radiative forcing relies significantly on the latitude change, its maximum value is located between 30°N and 90°N for both winter and summer, which shows that anthropogenic activities are the main cause for BC radiative forcing.